Merhaba Kübra, memnun oldum :)
The answer is both!
In the airport > specifically INSIDE, for example: "there are 1000 people working in the airport" ..they are inside the building, with a roof over their head
At the airport > generally there, but not under the airport's roof, for example: "There are 10 airplanes at the airport"
Sometimes you can use one to be more specific
Your friend calls you.
"Where are you? Are you in the airport? I'm outside by the taxi" = specifically IN the airport, under the roof
Your friend calls you
"Hi, did your flight arrive on time? Are you still at the airport? Or are you in the taxi?"
Summary: "In" = there is a roof over you or it
"At" = general term, not necessarily "inside"